The present invention is directed to bandgap reference circuits which provide a reference voltage independent of temperature. Specifically, a start-up circuit for a bandgap reference circuit is described whose operation is independent of power supply variations, which is an especially useful feature in battery operated circuits.
Bandgap reference circuits are used in numerous RF analog circuit applications to provide a very stable reference voltage. For example, a wireless, radio frequency (RF) telephone is typically operated with a battery capable of generating a raw voltage between 2.7 to 5.5 volts, depending upon its state of discharge. Many of the circuit elements comprising the telephone, such as a voltage controlled oscillator, cannot operate properly without a stable and accurate reference voltage. Although a bandgap reference circuit can provide an accurate and stable reference, it has its own operational deficiencies.
A bandgap reference circuit has two stable states, an off state, that does not provide a reference voltage, and an operational state, that does. When power is applied to a bandgap circuit, the circuit enters the off state, in which no current flows through the bandgap reference circuit. The bandgap reference circuit remains in the off state until an external circuit forces it to transition to the operational state. Once the operational state has been established, the external start-up circuit must be electrically disconnected from the bandgap reference circuit so that the start-up circuit no longer influences the circuit operation.
The invention provides a start-up circuit for a bandgap reference circuit that will perform its intended function irrespective of substantial voltage variations of a power supply. The invention is comprised of a bandgap reference circuit, a start-up circuit that is electrically connected to the bandgap reference circuit, and a supply-independent biasing circuit that is electrically connected to the start-up circuit. The start-up circuit forces the bandgap reference circuit to transition to its operational mode when power is supplied to the two circuits. A supply-independent biasing circuit is integrated with the start up circuit to provide the proper operation of the start-up circuit for any supply voltage that supports the bandgap reference circuit""s operational state.